Suspended germanium photodetector for silicon waveguide

ABSTRACT

A vertical stack of a first silicon germanium alloy layer, a second epitaxial silicon layer, a second silicon germanium layer, and a germanium layer are formed epitaxially on a top surface of a first epitaxial silicon layer. The second epitaxial silicon layer, the second silicon germanium layer, and the germanium layer are patterned and encapsulated by a dielectric cap portion, a dielectric spacer, and the first silicon germanium layer. The silicon germanium layer is removed between the first and second silicon layers to form a silicon germanium mesa structure that structurally support an overhanging structure comprising a stack of a silicon portion, a silicon germanium alloy portion, a germanium photodetector, and a dielectric cap portion. The germanium photodetector is suspended by the silicon germanium mesa structure and does not abut a silicon waveguide. Germanium diffusion into the silicon waveguide and defect density in the germanium detector are minimized.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/191,687, filed Aug. 14, 2008 the entire content and disclosure ofwhich is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a semiconductor structure, andparticularly to a germanium photodetector for a silicon waveguide, andmethods of manufacturing the same.

BACKGROUND OF THE INVENTION

Germanium photodetectors may be employed in microphotonic devices forthe high efficiency of photon absorption. Integration of a high quantumefficiency germanium photodetector into a silicon base semiconductorsubstrate faces a challenge because of the differences in materialproperty in silicon and germanium.

For example, Yin at al., “40 Gb/s Ge-on-SOI waveguide photodetectors byselective Ge growth,” Optical Fiber Communication/National Fiber OpticEngineers Conference, pp. 1-3, February, 2008, Digital Object Identifier10.1109/OFC.2008.4528025 discloses a silicon waveguide to which a Gephotodetector is attached. While the Ge photodetector in Yin provides anenhanced quantum efficiency over previous Ge photodetectors, theperformance of the Ge photodetector is limited by alloying of thegermanium material with the silicon material in the waveguide becausegermanium atoms have a high diffusivity in silicon. Since the photons inthe silicon waveguide may be scattered or reflected even by smallcrystalline defects or impurities, such a direct contact between thesilicon material in the waveguide and the germanium material in thephotodetector has an adverse impact on the quantum efficiency. The widerthe area of the contact between the silicon waveguide, the greater theamount of germanium atoms that diffuse into the silicon waveguide.

Further, silicon has a lattice constant of 0.543095 nm and germanium hasa lattice constant of 0.564613 nm at 300 K. The resulting latticemismatch of about 4% introduces severe strain on a germanium materialwhen the germanium material is grown epitaxially on a silicon material.Such a strain in the epitaxially grown germanium generates crystallinedefects, which generates a significant amount of dark current in thegermanium photodetector. The dark current is the electrical current thata photodetector generates in the absence of any signal, i.e., in theabsence of any light signal in the silicon waveguide. A high darkcurrent makes distinction between presence and absence of light signalin the silicon waveguide difficult.

In view of the above, there exists a need for a germanium photodetectorfor detecting light in a silicon waveguide with high quantum efficiencyand a minimal amount of dark current, and methods of manufacturing thesame.

Specifically, there exists a need for a germanium photodetector thatminimizes introduction of a germanium material into a silicon waveguideas well as minimizing crystalline defects in the germanium material ofthe photodetector, and methods of manufacturing the same.

SUMMARY OF THE INVENTION

The present invention provides a germanium photodetector that isevanescently coupled to a silicon waveguide and does not abut thesilicon waveguide.

In the present invention, a vertical stack of a first silicon germaniumalloy layer, a second epitaxial silicon layer, a second silicongermanium layer, and a germanium layer are formed epitaxially on a topsurface of a first epitaxial silicon layer. A dielectric cap layer isdeposited on the germanium layer. The vertical stack and the dielectriccap layer are lithographically patterned into a shape of a channelhaving long parallel edges. After forming a horizontal p-n junction inthe germanium layer, the stack of the dielectric cap layer, thegermanium layer, the second silicon germanium layer, and the secondsilicon layer are lithographically patterned in the shape of aphotodetector, which may have a tapered protrusion along the directionof the channel.

A dielectric spacer is formed on the sidewalls of the second siliconlayer, the second silicon germanium layer, and the germanium layer. Thedielectric spacer, the dielectric cap layer, and the silicon germaniumlayer encapsulate the photodetector. After removing the exposed portionsof the second silicon germanium layer, the second silicon germaniumlayer is undercut from beneath the second silicon layer to form asilicon germanium mesa structure that structurally support anoverhanging structure comprising a stack of a silicon portion, a silicongermanium alloy portion, a germanium photodetector, and a dielectric capportion. The remaining portion of the first silicon layer is a siliconwaveguide. The germanium photodetector is suspended by the silicongermanium mesa structure and overlies a cavity, which may be filled witha dielectric material layer or may be maintained as a cavity.

According to an aspect of the present invention, a semiconductorstructure is provided, which comprises:

a dielectric layer located on a substrate;

a silicon waveguide located on the dielectric layer and including aportion having a constant width and a constant height;

a silicon germanium mesa structure abutting an upper surface of thesilicon waveguide; and

a germanium photodetector located above the silicon germanium mesastructure and not abutting the silicon waveguide.

According to another aspect of the present invention, a method offorming a semiconductor structure is provided, which comprises:

forming a vertical stack including, from bottom to top, a first siliconlayer, a silicon germanium layer, a second silicon layer, and agermanium layer on a substrate, wherein all of the vertical stack issingle crystalline and epitaxially aligned among one another;

patterning the first silicon layer to form a silicon waveguide;

forming a photodetector including a p-n junction in the germanium layer;

forming a dielectric cap portion and a dielectric spacer directly on thephotodetector, wherein the dielectric cap portion, the dielectricspacer, and the second silicon layer encapsulates the photodetector; and

laterally removing the silicon germanium layer between the first siliconlayer and the second silicon layer, wherein a remaining portion of thesilicon germanium layer constitutes a silicon germanium mesa structure.

BRIEF DESCRIPTION OF THE DRAWINGS

For all of the figures herein, the following conventions apply. Figureswith the same numeric label correspond to the same stage ofmanufacturing in the same embodiment. Figures with the suffix “A” aretop-down views. Figures with the suffix “B” are horizontalcross-sectional views along the plane B-B′ in the figures with the samenumeric label and suffixes “D,” “E,” and “F.” Figures with the suffix“C” are horizontal cross-sectional views along the plane C-C′ in thefigures with the same numeric label and suffixes “D,” “E,” and “F.”Figures with the suffix “D,” “E,” or “F” are vertical cross-sectionalviews along the plane D-D′, E-E′, or F-F′, respectively, of thecorresponding figure with the same numeric label and the suffix “A”.

FIGS. 1A-7F are various views of a first exemplary semiconductorstructure according to a first embodiment of the present invention.

FIGS. 1A-1F correspond to the step after formation of a vertical stackof a first silicon germanium layer 30L, a second silicon layer 40, asecond silicon germanium layer 50, a germanium layer 60L, and adielectric cap layer 70 on a first silicon layer 20L on a substrate.

FIGS. 2A-2F correspond to the step after lithographic patterning of thevertical stack of the first silicon layer 20L, the first silicongermanium layer 30L, the second silicon layer 40, the second silicongermanium layer 50, the germanium layer 60L, and the dielectric caplayer 70 into a shape having substantially parallel sidewalls.

FIGS. 3A-3F correspond to the step after formation of a firstconductivity type germanium portion 60 and a second conductivity typegermanium portion 62.

FIGS. 4A-4F correspond to the step after patterning of the verticalstack of the first silicon germanium layer 30L, the first silicon layer40, the second silicon germanium layer 50, the germanium layer 60, andthe dielectric cap layer 70 into a shape having a taper in the directionparallel to the sidewalls of the first semiconductor layer 20L.

FIGS. 5A-5F correspond to the step after formation of a dielectricspacer 80 on the sidewalls of the second silicon layer 40, the secondsilicon germanium layer 50, the first-conductivity-type germaniumportion 60, and the second-conductivity-type germanium portion 62.

FIGS. 6A-6F correspond to the step after removal of portions of thefirst silicon germanium layer 30L to form a silicon germanium mesastructure 30.

FIGS. 7A-7F correspond to the step after formation of a first dielectricmaterial layer 90, a second dielectric material layer 92, at least onefirst contact via 94, and at least one second contact via 96.

FIGS. 8A-8F are various views of a second exemplary semiconductorstructure according to a second embodiment of the present invention. Acavity 97 laterally surrounding a silicon germanium mesa structure 30 isformed in the second exemplary semiconductor structure.

DETAILED DESCRIPTION OF THE INVENTION

As stated above, the present invention relates to a germaniumphotodetector for a silicon waveguide, and methods of manufacturing thesame, which are now described in detail with accompanying figures. Asused herein, when introducing elements of the present invention or thepreferred embodiments thereof, the articles “a”, “an”, “the” and “said”are intended to mean that there are one or more of the elements.Throughout the drawings, the same reference numerals or letters are usedto designate like or equivalent elements. Detailed descriptions of knownfunctions and constructions unnecessarily obscuring the subject matterof the present invention have been omitted for clarity. The drawings arenot necessarily drawn to scale.

Referring to FIGS. 1A-1F, a first exemplary semiconductor structureaccording to a first embodiment of the present invention comprises asemiconductor substrate, which may be a semiconductor-on-insulator (SOI)substrate. The semiconductor substrate includes an insulator layer 10and a first silicon layer 20L. The semiconductor substrate may furthercomprise a handle substrate (not shown) located directly beneath theinsulator layer 10. In this case, the first silicon layer 20L, theinsulator layer 10, and the handle substrate collectively constitute theSOI substrate.

The insulator layer 10 comprises a dielectric material such as siliconoxide, silicon nitride, silicon oxynitride, or a combination thereof.The thickness of the insulator layer 10 may be from about 50 nm to about10 m, and typically from about 200 nm to about 1 m, although lesser andgreater thicknesses are also contemplated herein.

The first silicon layer 20L comprises silicon. Preferably, the firstsilicon layer 20L consists essentially of silicon. The resistivity ofthe first silicon layer 20L is greater than about 1 Ohm-cm, andpreferably greater than about 10 Ohm-cm, and more preferably greaterthan about 100 Ohm-cm. The first silicon layer 20L may compriseamorphous silicon, polycrystalline silicon, or single crystallinesilicon.

Preferably, the first silicon layer 20L comprises a single crystallinesilicon, i.e., the entirety of the first silicon layer 20L is singlecrystalline with epitaxial alignment among all of the silicon atomstherein. Preferably, crystalline defects in the first silicon layer 20Lis kept as low as possible. In this case, the refractive index of thefirst silicon layer 20L in the infrared range may be about 3.45. Theabsorption constant of the single crystalline silicon in the firstsilicon layer 20L may be about 3.6 dB/cm, providing excellent lighttransmission characteristics.

The thickness of the first silicon layer 20L may be from about 50 nm toabout 1 m, and preferably from about 75 nm to about 500 nm, and morepreferably from about 100 nm to about 250 nm, although lesser andgreater thicknesses are also contemplated herein.

A first silicon germanium layer 30L is epitaxially grown directly on thetop surface of the first silicon layer 20L. In case the first siliconlayer 20L is single crystalline, the first silicon germanium layer 30Lis also single crystalline, and is epitaxially aligned to the firstsilicon layer 20L. The first silicon germanium layer 30L comprises asilicon germanium alloy.

In one case, the atomic concentration of the silicon germanium layer 30may be substantially constant. In another case, the composition of thefirst silicon germanium layer 30L may be vertically graded. The atomicconcentration of germanium may increase or decrease with distance fromthe interface between the first silicon layer 20L and the first silicongermanium layer 30L. In general, the first silicon germanium layer 30Lmay include a portion having a constant germanium concentration and/orat least another portion in which the atomic concentration of germaniumin the first silicon germanium layer 30L varies with the verticaldistance from the interface between the first silicon layer 20L and thefirst silicon germanium layer 30L.

The atomic concentration of germanium in the first silicon germaniumlayer 30L is non-zero, and may be from about 1% to about 99%, andtypically from about 5% to about 50%, although lesser and greater atomicconcentrations are also contemplated herein. Preferably, the atomicconcentration of germanium in the first silicon germanium layer 30L isselected to enable selective removal of the material of the firstsilicon germanium layer 30L relative to the silicon material of thefirst and second silicon layers (20L, 40) by an isotropic etch such as awet etch. The thickness of the first silicon germanium layer 30L may befrom about 30 nm to about 120 nm, and preferably from 40 nm to about 80nm, although lesser and greater thicknesses are also contemplatedherein.

A second silicon layer 40 is epitaxially grown directly on the topsurface of the first silicon germanium layer 30L. In case the firstsilicon layer 20L is single crystalline, the first silicon germaniumlayer 30L and the second silicon layer 40 are also single crystalline,and are epitaxially aligned to the first silicon layer 20L. The secondsilicon layer 40 comprises silicon. Preferably, the second silicon layer40 consists essentially of silicon. The thickness of the second siliconlayer 40 may be from about 10 nm to about 80 nm, and preferably from 15nm to about 60 nm, although lesser and greater thicknesses are alsocontemplated herein.

A second silicon germanium layer 50 is epitaxially grown directly on thetop surface of the second silicon layer 40. In case the first siliconlayer 20L is single crystalline, the first silicon germanium layer 30L,the second silicon layer 40, and the second silicon germanium layer 50are also single crystalline, and are epitaxially aligned to the firstsilicon layer 20L. The second silicon germanium layer 50 comprises asilicon germanium alloy. Preferably, the composition of the secondsilicon germanium layer 50 is vertically graded to reduce defectgeneration. Specifically, the atomic concentration of germanium maymonotonically increase with distance from the interface between thesecond silicon layer 40 and the second silicon germanium layer 50. Thesecond silicon germanium layer 50 may include a portion having aconstant germanium concentration, or the atomic concentration ofgermanium in the second silicon germanium layer 50 may strictly increasewith the vertical distance from the interface between the second siliconlayer 40 and the second silicon germanium layer 50.

Preferably, the atomic concentration of germanium in the second silicongermanium layer 50 changes from about 1% near the interface with thesecond silicon layer 40 to about 99% near the top surface of the secondsilicon germanium layer 50, although different germanium concentrationprofiles are also contemplated herein. The thickness of the secondsilicon germanium layer 50 may be from about 10 nm to about 80 nm, andpreferably from 15 nm to about 60 nm, although lesser and greaterthicknesses are also contemplated herein.

A germanium layer 60L is epitaxially grown directly on the top surfaceof the second silicon germanium layer 50. In case the first siliconlayer 20L is single crystalline, the first silicon germanium layer 30L,the second silicon layer 40, the second silicon germanium layer 50, andthe germanium layer 60L are also single crystalline, and are epitaxiallyaligned to the first silicon layer 20L. The germanium layer 60Lcomprises germanium. Preferably, the germanium layer 60L may bedeposited with in-situ doping, or may be subsequently doped withdopants, for example, by ion implantation, plasma doping, outdiffusionfrom a sacrificial dopant containing layer that is subsequently removed,or other equivalent methods. The type of doping in the germanium layer60L is herein referred to as a first conductivity type doping, which maybe a p-type doping or an n-type doping.

After the germanium layer 60L is doped with dopants of the firstconductivity type, the germanium layer 60L consists essentially ofgermanium and dopant atoms of the first conductivity type. In case thefirst conductivity type is p-type, the dopants may be boron, gallium,indium, or a combination thereof. In case the first conductivity type isn-type, the dopants may be phosphorus, arsenic, antimony, or acombination thereof. The atomic concentration of the dopants of thefirst conductivity type in the germanium layer 60L may be from about1.0×10¹⁶/cm³ to about 2.0×10²¹/cm³, and typically from about1.0×10¹⁸/cm³ to about 5.0×10²⁰/cm³, although lesser and greater dopantconcentrations are also contemplated herein.

The thickness of the germanium layer 60 may be from about 50 nm to about300 nm, and preferably from 100 nm to about 150 nm, although lesser andgreater thicknesses are also contemplated herein.

A dielectric cap layer 70 is deposited directly on the top surface ofthe germanium layer 60L. The dielectric cap layer 70 comprises adielectric material such as silicon nitride, silicon oxide, siliconoxynitride, or a combination thereof. The thickness of the dielectriccap layer 70 may be from about 5 nm to about 200 nm, and typically fromabout 20 nm to about 100 nm, although lesser and greater thicknesses arealso contemplated herein. The dielectric cap layer 70 may be formed bylow pressure chemical vapor deposition (LPCVD), plasma enhanced chemicalvapor deposition (PECVD), high density plasma chemical vapor deposition(HDPCVD), atomic layer deposition (ALD), etc.

Referring to FIGS. 2A-2F, a first photoresist 73 is applied overt thetop surface of the dielectric cap layer 70. The first photoresist 73 islithographically patterned in the shape of a line structure having twoparallel edges. The width w of the pattern in the first photoresist 73,which is the distance between the two parallel edges, may be from about150 nm to about 1.5 m, and preferably from about 200 nm to about 1.0 m,and more preferably from 300 nm to about 700 nm, although lesser andgreater widths w are also contemplated herein.

The pattern in the first photoresist 73 is transferred into the verticalstack, from bottom to top, of the first silicon layer 20L, the firstsilicon germanium layer 30L, the second silicon layer 40, the secondsilicon germanium layer 50, the germanium layer 60L, and the dielectriccap layer 70 by an anisotropic etch, which may be a reactive ion etch.The first photoresist 73 is employed as an etch mask. The sidewalls ofthe various layers are substantially vertically coincident in theremaining portion of the vertical stack (20L, 30L, 40, 50, 60L, 70)after the anisotropic etch, which has a constant width, i.e., the widthw of the pattern of the first photoresist 73, throughout.

Referring to FIGS. 3A-3F, a second photoresist 75 is applied over thevertical stack of the first silicon layer 20L, the first silicongermanium layer 30L, the second silicon layer 40, the second silicongermanium layer 50, the germanium layer 60L, and the dielectric caplayer 70. The second photoresist 75 is lithographically patterned tocover a portion of the vertical stack (20L, 30L, 40, 50, 60L, 70), whileexposing another portion of the vertical stack (20L, 30L, 40, 50, 60L,70). For example, an edge of the second photoresist 75 may run acrossthe entirety of the top surface of the dielectric cap layer 70 at rightangle from the direction of the parallel edges of the vertical stack(20L, 30L, 40, 50, 60L, 70) separated by the width w.

Dopants of a second conductivity type are implanted into an upperportion of the germanium layer 60L which has a doping of the firstconductivity type. The second conductivity type is the opposite of thefirst conductivity type. For example, if the first conductivity type isp-type, the second conductivity type is n-type, and vice versa. The doseof the ion implantation is selected so that the concentration of thesecond conductivity dopants in the implanted region exceeds theconcentration of the first conductivity dopants. Thus, asecond-conductivity-type germanium region 62 having a net doping of thesecond conductivity type is formed in an upper portion of the germaniumlayer in the exposed area, i.e., in the area not covered by the secondphotoresist 75. The net doping concentration, i.e., the concentration ofthe second conductivity dopants less the concentration of the firstconductivity dopants, in the second-conductivity-type germanium region62 may be from about 1.0×10¹⁶/cm³ to about 2.0×10²¹/cm³, and typicallyfrom about 1.0×10¹⁸/cm³ to about 5.0×10²⁰/cm³, although lesser andgreater dopant concentrations are also contemplated herein.

The energy and species of the dopants of the second conductivity typethat are implanted into the second-conductivity-type germanium region 62is selected so that the bottom surface of the second-conductivity-typegermanium region 62 is formed between the top surface and the bottomsurface of the germanium layer 60L. The remaining portions of thegermanium layer 60L underneath the second-conductivity-type germaniumregion 62 or underneath the second photoresist 75 has the same doping asbefore the ion implantation of the second conductivity dopants, and areherein collectively referred to as a first-conductivity-type germaniumregion 60. One of the first-conductivity-type germanium region 60 andthe second-conductivity-type germanium region 62 is a p-doped germaniumportion, and the other of the first-conductivity-type germanium region60 and the second-conductivity-type germanium region 62 is an n-dopedgermanium portion.

A p-n junction is formed at the interface between thefirst-conductivity-type germanium region 60 and thesecond-conductivity-type germanium region 62. Thus, thefirst-conductivity-type germanium region 60 and thesecond-conductivity-type germanium region 62 collectively function as aphotodetector. The first-conductivity-type germanium region 60 and thesecond-conductivity-type germanium region 62 are collectively referredto as a germanium photodetector (60, 62). The p-n junction may include asubstantially horizontal interface between the first-conductivity-typegermanium region 60 and the second-conductivity-type germanium region62. Depletion regions are formed on both sides of the p-n junction.Preferably, horizontal surfaces of the depletion regions do not abut thetop surface of the second-conductivity-type germanium region 62 or thebottom surface of the first-conductivity-type germanium region 60. Thesecond photoresist 75 is subsequently removed.

Referring to FIGS. 4A-4F, a third photoresist 77 is applied over thefirst exemplary semiconductor structure and is lithographicallypatterned to form a shape having at least two different widths in thedirection perpendicular to the sidewalls of the first silicon layer 20L.The third photoresist 77 as patterned covers a portion of the verticalstack of the of the first silicon layer 20L, the first silicon germaniumlayer 30L, the second silicon layer 40, the second silicon germaniumlayer 50, the germanium detector (60, 62), and the dielectric cap layer70, and may cover adjoining portions of the insulator layer 10.Preferably, the pattern in the third photoresist 77 includes a taperedportion having a monotonically decreasing width along the direction ofthe sidewalls of the first silicon layer 20L. The pattern in the thirdphotoresist 77 is transferred into the stack, from top to bottom, of thedielectric cap layer 70, the germanium photodetector (60, 62) which is agermanium layer, the second silicon germanium layer 50, and the secondsilicon layer 40 by an anisotropic etch, which may be a reactive ionetch. The anisotropic etch stops on the top surface of the first silicongermanium layer 30L or in the middle of the first silicon germaniumlayer 30L. The anisotropic etch may be selective to the material of thefirst silicon germanium layer 30L. Not necessarily but preferably, theanisotropic etch is also selective to the material of the insulatorlayer 10.

All layers in the patterned stack of the second silicon layer 40, thesecond silicon germanium layer 50, the germanium detector (60, 62), andthe dielectric cap layer 70 have substantially vertically coincidentsidewalls. The patterned stack (40, 50, 60, 62, 70) may include aconstant width portion, which has the same width as the first siliconlayer 20L, and a tapered portion that has a monotonically decreasingwidth with distance from the constant width portion in a directionparallel to the sidewalls of the first silicon layer 20L. The taperedportion may have a strictly decreasing width with distance from theconstant width portion of the patterned stack (40, 50, 60, 62, 70).

As used herein, a monotonic decrease in width with distance denotes thatthe a first width at a first distance is not more than a second width ata second distance if the first distance is greater than the seconddistance for any pair of the first and second distances. As used herein,a strict decrease in width with distance denotes that a first width at afirst distance is less than a second width at a second distance if thefirst distance is greater than the second distance for any pair of thefirst and second distances. In one example, the tapered portion may havea constant taper, i.e., the rate of decrease in the width of the taperedportion with the distance is constant throughout the tapered portion. Inanother example, the width of the tapered portion may become zero at adistal end of the tapered portion, i.e., the cross-sectional area of thetapered portion may have a pointed end. The third photoresist 77 issubsequently removed.

Referring to FIGS. 5A-5C, dielectric spacers are formed on the sidewallsof the first exemplary semiconductor structure, which include sidewallsof the first silicon layer 20L and the first silicon germanium layer 30Land sidewalls of the patterned stack (40, 50, 60, 62, 70) that include,from bottom to top, the second silicon layer 40, the second silicongermanium layer 50, the germanium detector (60, 62), and the dielectriccap layer 70. The dielectric spacers are formed by deposition of aconformal dielectric material layer, followed by an anisotropic etchthat removes horizontal portion of the conformal dielectric materiallayer. The vertical portions of the conformal dielectric material layerthat remains on the sidewalls of the patterned stack (40, 50, 60, 62,70) after the anisotropic etch constitutes a first dielectric spacer 80,and the vertical portions of the conformal dielectric material layerthat remains on the sidewalls of the first silicon layer 20L and thefirst silicon germanium layer 30L after the anisotropic etch constitutesa second dielectric spacer 82. In case the constant thickness portion ofthe patterned stack (40, 50, 60, 62, 70) have a width comparable to thewidth of the first silicon layer 20L, the first dielectric spacer 80 andthe second dielectric spacer 82 may be of unitary and integralconstruction, i.e., formed as a single contiguous structure without anyinterface therebetween.

The first dielectric spacer 80 and the second dielectric spacer 82comprise a dielectric material such as silicon nitride, silicon oxide,silicon oxynitride, or a combination thereof. The dielectric materialhas a lower refractive index than the refractive index of silicon, whichis about 3.45. For example, silicon oxide has a refractive index ofabout 1.45, and silicon nitride has a refractive index of about 2.05. Byinsuring that the refractive index of the second dielectric spacer 82 isless than the refractive index of silicon, light may be confined in asilicon waveguide which is derived from the first silicon layer 20L bytotal reflection at the walls of the silicon waveguide. The lateralthickness of the first dielectric spacer 80 and the second dielectricspacer 82 may be from about 5 nm to about 200 nm, and preferably fromabout 20 nm to about 100 nm, although lesser and greater thicknesses arealso contemplated herein.

The first dielectric spacer 80 laterally abuts and laterally surrounds aportion (50, 60, 62) of the patterned stack (40, 50, 60, 62, 70). Thisportion (50, 60, 62) of the patterned stack (40, 50, 60, 62, 70) isvertically bound by the dielectric cap layer 70 at the top and thesecond silicon layer 40 at the bottom. The second silicon germaniumlayer 50 and the germanium photodetector (60, 62), which is a germaniumlayer, are encapsulated by the dielectric cap layer 70, the firstdielectric spacer 80, and the second silicon layer 40. Particularly, thegermanium photodetector (60, 62) is encapsulated by the dielectric caplayer 70, the first dielectric spacer 80, and the second silicongermanium layer 50. The second silicon germanium layer 50 isencapsulated by the germanium photodetector (60, 62), the firstdielectric spacer 80, and the second silicon layer 40.

Referring to FIGS. 6A-6F, the exposed portions of the first silicongermanium layer 30L are removed by an isotropic etch, which may be a wetetch. For example, a wet etch chemistry that contains ammonium hydroxide(NH₄OH) and hydrogen peroxide (H₂O₂) may be employed to remove thesilicon germanium alloy in the first silicon germanium layer 30L. Thehigher the germanium concentration in the first silicon germanium layer30L, the more effective this etch chemistry is in removing the materialof the first silicon germanium layer 30L. Further, the etch is extendedto induce removal of the material of the first silicon germanium layer30L between the first silicon layer 20L and the second silicon layer 40.Thus, the material of the first silicon germanium layer 30L is laterallyundercut inward from the peripheral surfaces that coincide with thesidewalls of the patterned stack (40, 50, 60, 62, 70).

The lateral undercut of the first silicon germanium layer 30L is stoppedbefore the entirety of the material in the first silicon germanium layer30L is removed by the isotropic etch. A portion of the first silicongermanium layer 30L remains underneath the region of the constantthickness portion, or a portion having the greatest width, in thepatterned stack (40, 50, 60, 62, 70). The remaining portion of the firstsilicon germanium layer 30L is herein referred to as a silicon germaniummesa structure 30. The width of the silicon germanium mesa structure isless than the width of the first silicon layer 20L. Preferably, thematerial of the first silicon germanium layer 30L is removed fromunderneath the tapered portion of the patterned stack (40, 50, 60, 62,70).

The material of the first silicon germanium layer 30L is removed fromthe top surface of the first silicon layer 20L outside the area thatunderlies the constant thickness portion of the patterned stack (40, 50,60, 62, 70). The first silicon layer 20L may be employed as an effectivesilicon waveguide that transmits light without any significant signalloss at this point. Therefore, the first silicon layer 20L is alsoreferred to as a silicon waveguide 20 hereafter.

In case the first dielectric spacer 80 and the second dielectric spacer82 may be of unitary and integral construction, two tunnel cavities maybe formed between the silicon waveguide 20 and the second silicon layer40 and between the silicon germanium mesa structure 30 and the seconddielectric spacer 82. The patterned stack (40, 50, 60, 62, 70) hangsover the silicon waveguide 20. The patterned stack (40, 50, 60, 62, 70)including the germanium detector (60, 62) overlies the entirety of thesilicon germanium mesa structure 30, and a portion of the germaniumdetector (60, 62) does not overlie the silicon germanium mesa structure30. As seen in a see-through top-down view, the periphery of the silicongermanium mesa structure 30 is entirely contained within the peripheryof the germanium detector (60, 62).

Referring to FIGS. 7A-7F, a first dielectric material layer 90 and asecond dielectric material layer 92 are formed on the first exemplarysemiconductor structure. For example, the material of the firstdielectric material layer 90 may be formed by conformal deposition, andmay fill the entirety of the gap between the silicon waveguide 20 andthe second silicon layer 40. Further, the first dielectric layer coversthe entirety of the exposed surfaces of the first and second dielectricspacers (80, 82) and the dielectric cap layer 70. The thickness of thefirst dielectric material layer 90 is greater than the thickness, or theheight, of the silicon germanium mesa structure to enable the filling ofthe gap between the silicon waveguide 20 and the second silicon layer40.

The dielectric materials that may be used for the first dielectricmaterial layer 90 and the second dielectric material layer 92 include,but are not limited to, a silicate glass, an organosilicate glass (OSG)material, a SiCOH-based low-k material formed by chemical vapordeposition, a spin-on glass (SOG), or a spin-on low-k dielectricmaterial such as SiLK™, etc. The silicate glass includes an undopedsilicate glass (USG), borosilicate glass (BSG), phosphosilicate glass(PSG), fluorosilicate glass (FSG), borophosphosilicate glass (BPSG),etc. The dielectric material may be a low dielectric constant (low-k)material having a dielectric constant less than 3.0. The dielectricmaterial may non-porous or porous. The dielectric material for the firstdielectric material layer 90 has a lower refractive index than silicon.

At least one contact via 94 that vertically abuts thefirst-conductivity-type germanium region 60 and at least another contactvia 96 that vertically abuts the second-conductivity-type germaniumregion 62 are formed through the first dielectric material layer 90 andthe second dielectric material layer 92. The at least one contact via 94and the at least another contact via 96 comprise a conductive materialsuch as W, Cu, Al, TaN, TiN, Ta, Ti, or a combination thereof.

Referring to FIGS. 8A-8F, a second exemplary semiconductor structureaccording to a second embodiment of the present invention is derivedfrom the first exemplary semiconductor structure of FIGS. 6A-6F. In thesecond embodiment, the first dielectric material layer 90 is formed by anon-conformal deposition method. The gap between the silicon waveguide20 and the second silicon layer 40 is not completely filled. Due to thenon-conformal nature of the deposition process, the patterned stack (40,50, 60, 62, 70) prevents deposition of the material of the firstdielectric material layer 90 at least in portions of the gap between thesilicon waveguide 20 and the second silicon layer 40. The unfilled gapbecomes a cavity 97, which laterally surrounds the silicon germaniummesa structure 97. The topological structure of the cavity 97 may be thesame as the topological structure of a torus, i.e., the cavity 97 may betopologically homeomorphic to a torus so that the shape of the cavity 97may be transformed into the shape of a torus without forming ordestroying a topological singularity, i.e., a new hole in the shape.

The first exemplary semiconductor structure of FIGS. 7A-7F or the secondexemplary semiconductor structure of FIGS. 8A-8F may be employed as acombination of a silicon waveguide 20 and a germanium photodetector (60,62). The germanium detector (60, 62) is not located directly on thesilicon waveguide 20. However, evanescent coupling, which is a quantumeffect due to the quantum electrodynamic property of light, enablescoupling and detection of light in the silicon waveguide 20 by thegermanium photodetector (60, 62).

The light may have a wavelength from about 400 nm to about 1,700 nm invacuum, and have a wavelength that is shortened by a factor that is thesame as the refractive index of silicon, i.e., by a factor of about3.45, in the silicon waveguide. Because the material outside the siliconwaveguide 20 has a lower refractive index than silicon, total reflectionoccurs within the waveguide 20 so that the loss of light along thedirection of the silicon waveguide is solely due to absorption of thelight. Since the absorption coefficient of silicon is about 3.6 dB/cm insingle crystalline silicon, the light is transmitted through the siliconwaveguide 20 with little loss in intensity.

The photodetector (60, 62) coupled with light that travels along thesilicon waveguide 20 by optically coupling with evanescent portion ofthe light wave. Although the light is nominally “confined” within thesilicon waveguide 20, the wave property of light causes the wavefunctionof the light to extend outside the silicon waveguide 20. The evanescentportion of the wavefunction of the light decays exponentially outsidethe silicon waveguide 20 with the distance from the sidewalls of thesilicon waveguide 20. The effective range of the evanescent portion ofthe wavefunction may be from about 150 nm to about 600 nm, depending onthe wavelength of the light and the dimensions of the silicon waveguide20. Once the light couples with the photodetector (60, 62), the energyof the light is absorbed by the photodetector (60, 62) as a lightparticle.

Upon interaction with light, the photodetector (60, 62) generateselectron-hole pairs as a photodiode. Charge carriers of the secondconductivity type are collected in the second-conductivity-typegermanium region 62 in proportion to the amount of photons that interactwith the photodetector (60, 62). In case the first conductivity type isp-type and the second conductivity type is n-type, electrons arecollected in the second-conductivity-type germanium region 62. In casethe second conductivity type is n-type and the second conductivity typeis p-type, holes are collected in the second-conductivity-type germaniumregion 62.

If the electron-hole pair is generated within the depletion region ofthe photodetector (60, 62), which extends into a portion of thesecond-conductivity-type germanium region 62 and a portion of thefirst-conductivity-type germanium region 60 from the p-n junction, thecharge carriers (holes and electrons) drift apart due to the kineticenergy imparted to the charge carriers during the photogenerationprocess. If a minority carrier (a charge carrier of the firstconductivity type in the second-conductivity-type germanium region 62 ora charge carrier of the second conductivity type in thefirst-conductivity-type germanium region 60) enters into the depletionregion by drifting, the electric field inherent in the depletion regionof the photodetector (60, 62) sweeps the carrier across the p-njunction, which then becomes a majority carrier, i.e., a charge carrierof the first conductivity type in the first-conductivity-type germaniumregion 60 or a charge carrier of the second conductivity type in thesecond-conductivity-type germanium region 62, upon crossing the p-njunction, and producing a photocurrent if the circuit is closed, oraccumulates charges.

The photocurrent flows through the at least one contact via 94 and theat least another contact via 96 to a sensing circuit, which may beformed on the semiconductor substrate which may be formed in anotherportion of the first silicon layer 20L that is not patterned into asilicon waveguide 20. The presence of current through the sensingcircuit signifies presence of the light signal in the silicon waveguide20, and the absence of current through the sensing circuit signifies theabsence of any light signal in the silicon waveguide 20. Thus, anoptical signal is converted to an electrical signal by the germaniumphotodetector (60, 62).

The tapered portion of the photodetector (60, 62) maximizes the couplingof the photodetector (60, 62) with the evanescent portion of the lightby gradually changing the effective refractive index of the regionpermeated by the evanescent portion of the light. Preferably, the lengthof the tapered portion of the photodetector (60, 62) is at least as longas the wavelength of the light, and is preferably at least several timesgreater than the wavelength of the light in the silicon waveguide 20.For example, the length of the tapered portion of the photodetector (60,62) may be from about 1 m to about 30 m, and preferably from about 3 mto about 10 m, although lesser and greater lengths are also contemplatedherein.

The combination of the silicon waveguide 20 and the germaniumphotodetector (60, 62), as shown in FIGS. 7A-7F and 8A-8F, providesuperior performance over prior art structures in many aspects. First,the contact area between the silicon germanium mesa structure 30 and thesilicon waveguide 20 may be much less than the planar area of thegermanium photodetector (60, 62). Since the silicon waveguide 20 makesless areal contact with a germanium-containing material, less germaniumdiffuses into the silicon waveguide 20, thereby keeping the material ofthe silicon waveguide 20 pure with less impurities, and thereby reducingloss of the light signal either by reflection or absorption.

Second, the crystalline defect density in the silicon germanium mesastructure 30 and the patterned stack (40, 50, 60, 62, 70) including thegermanium photodetector (60, 62) may be decreased since the abruptcompositional transition that is required for prior art germaniumdetectors that are formed directly on a silicon waveguide is replaced bya gradual compositional change between successive epitaxial layers.Particularly, the second silicon germanium layer 50 allows gradualchange of the atomic concentration of germanium, thereby enablingepitaxial growth of a high quality low defect density germanium materialin the germanium layer 60L that is employed for the germaniumphotodetector. Thus, the dark current in the germanium photodetector(60, 62) is reduced with the decrease in the crystalline defect densityin the material of the germanium photodetector (60, 62).

Third, the tapered portion of the patterned stack (40, 50, 60, 62, 70)provides a gradual change in the effective refractive index that theevanescent portion of the light in the silicon waveguide detects. Forthe evanescent portion of the light, the effective refractive indexchanges from the effective refractive index of the first and seconddielectric material layers (990, 92), which may be from about 1.45 to2.05, to the refractive index of germanium, which is about 4.0. Sincethe reflection of the light is minimized by the gradual change of therefractive index as seen by the light, the intensity of light impingingon the germanium photodiode (60, 62) increases.

Such advantageous features of the present invention are combined toprovide high quantum efficiency for light detection and low dark currentto the germanium detector (60, 62) of the present invention.

While the invention has been described in terms of specific embodiments,it is evident in view of the foregoing description that numerousalternatives, modifications and variations will be apparent to thoseskilled in the art. Accordingly, the invention is intended to encompassall such alternatives, modifications and variations which fall withinthe scope and spirit of the invention and the following claims.

1. A method of forming a semiconductor structure comprising: forming avertical stack including, from bottom to top, a first silicon layer, asilicon germanium layer, a second silicon layer, and a germanium layeron a substrate, wherein all of said vertical stack is single crystallineand epitaxially aligned among one another; patterning said first siliconlayer to form a silicon waveguide; forming a photodetector including ap-n junction in said germanium layer; forming a dielectric cap portionand a dielectric spacer directly on said photodetector, wherein saiddielectric cap portion, said dielectric spacer, and said second siliconlayer encapsulates said photodetector; and laterally removing saidsilicon germanium layer between said first silicon layer and said secondsilicon layer, wherein a remaining portion of said silicon germaniumlayer constitutes a silicon germanium mesa structure.
 2. The method ofclaim 1, wherein said silicon germanium mesa structure vertically abutsa top surface of said silicon waveguide and a bottom surface of saidsecond silicon layer, wherein said photodetector includes a firstportion that overlies an entirety of said silicon germanium mesastructure and a second portion that does not overlie said silicongermanium mesa structure.
 3. The method of claim 1, further comprising:patterning said first silicon layer into a shapes having a constantwidth between a pair of substantially vertical sidewalls, wherein saidpatterned first silicon layer constitutes said silicon waveguide; andpatterning said germanium layer into a shape having at least twodifferent lateral widths.
 4. The method of claim 3, wherein said shapeof said germanium layer includes a tapered portion in which a lateralwidth of said tapered portion monotonically decreases along a lengthwisedirection of said silicon waveguide.
 5. The method of claim 1, furthercomprising forming another silicon germanium layer directly on a topsurface of said second silicon layer, wherein said germanium layer isformed directly on a top surface of said other silicon germanium layer,wherein said dielectric cap portion, said dielectric spacer, and saidother silicon germanium layer encapsulates said photodetector.
 6. Themethod of claim 1, further comprising forming a dielectric materiallayer laterally abutting and laterally surrounding sidewalls of saidsilicon germanium mesa structure and vertically abutting a top surfaceof said silicon waveguide and a bottom surface of said second siliconlayer.
 7. The method of claim 1, further comprising forming a cavitylaterally abutting and laterally surrounding sidewalls of said silicongermanium mesa structure and vertically abutting a top surface of saidsilicon waveguide and a bottom surface of said second silicon layer.